This invention pertains to antireflection coatings for silicon solar cells and more particularly to such coatings incorporating the altered layer formed on polycrystalline silicon as a result of hydrogen ion-beam passivation.
As is well known, cost considerations make it desirable to fabricate silicon solar cells from polycrystalline, rather than monocrystalline, silicon. However, because of the minority carrier losses at grain boundaries, dislocations, and the like, the efficiencies realized with polycrystalline silicon solar cells are generally poorer than those of monocrystalline cells. This circumstance has been improved upon by introducing a monovalent element, such as hydrogen, into the polycrystalline material to combine with the dangling bonds associated with the structural defects, thereby minimizing the minority carrier recombination loss.
A variety of manufacturing protocols have been developed to integrate a hydrogen passivation step into the volume manufacture of solar cells. For instance, as taught in U.S. Patent applications Ser. No. 681,003, filed 13 Dec. 1984 (which is a continuation-in-part of U.S. patent application Ser. No. 659,279, filed 10 Oct. 1984, abandoned, which is a continuation of U.S. patent application Ser. No. 563,061, filed 19 Dec. 1983, abandoned), and U.S. patent application Ser. No. 681,001, filed 13 Dec. 1984 (which is a continuation-in-part of U.S. patent application Ser. No. 666,972, filed 31 Oct. 1984, abandoned, which is a continuation of U.S. patent application Ser. No. 563,292, filed 19 Dec. 1986, abandoned), and U.S. Pat. No. 4,557,037 (issued on U.S. patent application Ser. No. 681,498, filed 13 Dec. 1984, which is a continuation-in-part of U.S. patent application Ser. No. 666,973, filed 31 Oct. 1984, abandoned, which is a continuation of U.S. patent application Ser. No. 563,132, filed 19 Dec. 1983, abandoned) the hydrogen passivation may be. incorporated into the manufacturing process so as to form a plating mask for the control of subsequent metallization of the front surface electrodes. Thus, as described in U.S. patent application Ser. No. 681,003 a preferred embodiment of the process described in detail therein as applied to the manufacture of silicon solar cells involves, inter alia, the following steps: (1) forming a plating mask of a dielectric material on the front surface of a shallow-junction silicon ribbon so as to leave exposed those areas of the silicon to be covered by the front surface electrode, (2) depositing a thin layer of nickel (or similar material) on the exposed silicon, thereby forming an initial metal layer in the electrode pattern, (3) removing the plating mask, thereby exposing the silicon between the initial metal layer of the front electrode, (4) hydrogen ion-beam passivating the junction side of the cell in such a way as to form, inter alia, a fresh plating mask on the silicon between the electrodes, (5) sintering the nickel to form in part a nickel silicide, (6) plating additional metal(s) onto the metal-covered portions of the cell, and (7) applying or forming an antireflection coating on the exposed surface of the silicon. Thereafter, the silicon may be further processed, e.g., to prepare it for connection to electrical circuits. In an alternative process, the heating of the sample during passivation supplies at least part of the energy for the nickel sintering step. Related proceedures are taught in U.S. patent applications Ser. Nos. 681,001 and 681,498 (now U.S. Pat. No. 4,557,037).
A number of other ion-beam passivation proceedures are also known, and it should be noted that the passivation step need not be used to form a plating mask. In any event, it has been found that, as a result of hydrogen ion-beam passivation, an altered surface layer is formed on the silicon (for the proceedures cited hereinbefore, this altered layer is used as the subsequent plating mask), and that this layer has a refractive index lower than that of untreated silicon. Accordingly, the refractive index of an ideal antireflection coating that overcoats such a layer should also be lower than for virgin silicon.
For example, as is well known, the refractive index, n.sub.1, of a quarter wave antireflection coating should relate to the indices n.sub.0 and n.sub.2 of the materials on either side of it by the relationship EQU n.sub.1.sup.2 =n.sub.0 n.sub.2
For such an antireflection coating separating air and silicon, n.sub.0 =1.000 and n.sub.2 is the index of refraction of the silicon. For virgin silicon, n.sub.2 =3.78 at 633 nm, and consequently, a quarter wave antireflection coating having an index of 1.95 is ideal. It is well known that such an index may be achieved with a silicon nitride coating, which, by appropriate plasma deposition protocols, may be made to have refractive indices between about 1.85 and 1.95. Thus, virgin silicon may be easily optically matched to air with a practical coating procedure.
In contrast, the altered layer formed as a result of hydrogen ion-beam passivation has a refractive index much lower, values as low as 3.17 at 633 nm having been observed. An ideal quarter wave antireflection coating for air would accordingly have an index of 1.78, well below the index achievable with silicon nitride. If a silicon nitride antireflection coating were used, there would be an optical mismatch, and the efficiency gains made through the minimization of the minority carrier recombination losses by hydrogen passivation would be partially offset by increased Fresnel losses.
A possible solution would be to use other, less desirable, antireflection coatings having lower refractive indices. However, the situation is exacerbated since it is also found that the effective refractive index of the altered layer formed in ion-beam passivation varies from sample to sample.
Stripping the altered layer from the workpiece, as by etching, is another possible approach. However, the additional processing step is not desirable in a production process.
It might also be noted that while adding an antireflection coating to a cell is desirable from the standpoint of cell performance, it is an additional step, and therefore an additional expense, in the manufacturing process.